The present invention relates to Schottky diodes and in particular, Schottky diodes with planar geometry and the method of making same.
A Schottky diode is a rectifying metal semiconductor junction. This diode is used in microwave circuits as a mixer where a local oscillator waveform is combined with an incoming signal to produce a difference frequency known as the intermediate frequency.
The performance of the Schottky diode is typically gaged by the product of its series resistance and its junction capacitance. This product must be minimized for the best performance to be achieved.
A high conductivity active layer is used in the Schottky diode to reduce the series resistance, but this also increases the capacitance. The capacitance is reduced by making the Schottky contact itself as small as possible. In general, the area of these contacts is less than 10.sup.-7 cm.sup.2. Because of the size of the Schottky contact,fabrication of the Schottky diode has always been a problem.
The whisker diode, a particular type of Schottky diode, was fabricated by producing a matrix of holes in an oxide deposited on the semiconductor and dropping a whisker at random in one hole and holding it by contact pressure. There were several disadvantages with this structure. The parasitic capacitance was high, it was not mechanically rugged being adversely affected by humidity and mechanical shock, and fabrication was both difficult and expensive.
The planar mixer diode, another type of Schottky diode, is more reliable and rugged than the whisker diode. It consists of a high conductivity n.sup.+ layer and n layer grown consecutively on a gallium arsenide substrate by any epitaxial method. The Schottky contact itself is a narrow metal finger typically of titanium-tungsten. The Schottky contact is placed close to and is surrounded by the ohmic contact which is typically made of gold-germanium. The capacitance is reduced by making the Schottky contact as small as possible. It often has an area as small as one micrometer to ten micrometers. The stray capacitance is reduced by making the diode semi-insulating by bombarding it with protons while leaving an active region in the vicinity of the contacts. Registration and fabrication of this device is also difficult and expensive because of the size of the Schottky contact and its closeness to the ohmic contact.